In recent years, a CIS-based thin film solar cell that uses a chalcopyrite structure I-III-VI2 group compound semiconductor containing Cu, In, Ga, Se and S as a p-type light absorbing layer has been attracting attention. Since a solar cell of this type can be manufactured at relatively low cost and is expected to achieve high photoelectric conversion efficiency, it is widely considered a leading candidate for a next-generation solar cell. Typical materials include Cu(In, Ga)Se2, Cu(In, Ga)(Se, S)2, CuInS2, etc.
In a CIS-based thin film solar cell, a metal backside electrode layer is formed on a glass substrate. A p-type light absorbing layer comprised of a I-III-VI2 group compound semiconductor is formed on the backside electrode layer, and then a buffer layer and a window layer are formed. In this CIS-based thin film solar cell, it has been reported that high photoelectric conversion efficiency can be achieved when soda lime glass is used as the glass substrate. This is because Na, which is an Ia group element, contained in the soda lime glass is diffused into the p-type light absorbing layer in the formation process of this layer and affects carrier concentration. Therefore, in a CIS-based thin film solar cell, it has been known that controlling of the introduction of Na into the p-type light absorbing layer is an important task that is necessary to improve its photoelectric conversion efficiency.
Controlling the introduction of Na into a p-type light absorbing layer are broadly divided into two categories. The first method of control utilizes the fact that Na contained in a soda lime glass substrate is diffused and absorbed into the p-type light absorbing layer in the formation process of the CIS-based p-type light absorbing layer and controls the amount of diffusion. (See Patent Literature 1.) The second one adds a Na compound from the outside in the formation process of the p-type light absorbing layer. In this case, after inhibiting the diffusion of Na from the glass substrate by providing a blocking layer between the glass substrate and the p-type light absorbing layer, or ensuring that Na is not diffused from the substrate by using the glass substrate not containing Na, the Na compound is added to the p-type light absorbing layer. By doing so, the Na concentration in the p-type light absorbing layer is controlled. (See Patent Documents 2 and 3 and Non-patent Literature 1.)
The first method described above utilizes soda lime glass as the glass substrate. However, soda lime glass has a problem in that it has a relatively low strain point and deforms if the p-type light absorbing layer is formed at a high temperature, for example, at 550° C. or more to improve photoelectric conversion efficiency, and therefore formation temperature cannot be increased. In order to carry out the formation process at such high temperature, a low alkali glass, such as high strain point glass or non-alkali glass has to be used as the glass substrate. However, such glass contains little or no alkali and cannot supply a sufficient amount of alkali to the p-type light absorbing layer.
The second method does not use soda lime glass, and therefore it can solve the problem of the first method described above. However, in this method, it is difficult to add alkali to the p-type light absorbing layer uniformly and with good lot-to-lot reproducibility. It is difficult to handle an alkali metal, such as Na and in order to add an alkali metal to the p-type light absorbing layer, a stable compound, such as NaF, has to be added by spraying or mixed into the Se material. Efficiency of such addition is poor and in the case of NaF, F may adversely affect the formation of the p-type light absorbing layer. Further, when NaF is added to the p-type light absorbing layer by spraying and the like, the diameters of the added particles may not be uniform, and therefore uniform spraying is difficult.